I'd like to better understand some concepts from Aristotle's Metaphysics. I'd like to know what does he mean by a "substance" (Ousia), does he just mean anything that exists? Can any substance be "explained" by Four Causes? For example, does a seed (of a fruit) answer all four causes? Well, it has a matter, it's effective cause would be (from what I understand) soil, water, warmth... And it's final cause would be a tree? But I'm having trouble grasping what would the formal cause of a seed be?
I'd also like to know how are the Four Causes and Aristotle's principles of "Potentiality and Actuality" connected? From what I gather, the "final cause" is just an actuality of a substance, while a "formal cause" has that potentiality?
For some context: At my university Philosophy is a mandatory course, and we only covered "Metaphysics" from Aristotle, so I might be missing out on some concepts he might have explained elsewhere (though I've tried googling). Please do answer my questions and / or point me to something a beginner can understand. Thanks!